Conventionally, a silicone foam or a rubber sponge is used as a sealing material around a heat source part, for example, around a battery of an electric vehicle or around an electron control part, or in a sealing portion of a solar cell.
However, a sealing material comprising a silicone foam was difficult to be formed so as to have a thickness less than 0.8 mm. For this reason, the sealing material comprising a silicone foam has large thickness, and could not be used in a region requiring a thin sealing material. Furthermore, raw material cost is high, and a product could not be provided at a low price.
On the other hand, a rubber sponge has large compressive residual strain, and therefore has low long-term sealing property. Additionally, there is a possibility that a gas containing sulfur, generated from a rubber sponge corrodes an electronic substrate.
Furthermore, it is proposed that a polyurethane foam obtained by a mechanical froth method is used as a sealing material (Patent Documents 1 and 2).
The mechanical froth method is that first of all, as a polyurethane raw material, a foam-forming gas is compressed and mixed with a raw material containing polyol, isocyanate, a foam stabilizer and a catalyst. The polyurethane raw material is discharged from an oaks mixer or a nozzle having a tapered tip to form a polyurethane foam. The gas for foam formation compressed until then expands when discharging the polyurethane raw material and forms bubbles, and polyol is reacted with isocyanate under such a state to cure the polyurethane raw material, thereby foaming a polyurethane foam.
The polyurethane foam produced by a mechanical froth method is formed into a thin sheet state suitable as a sealing material. The polyurethane foam itself is inexpensive than a silicone foam, and additionally has good compressive residual strain as compared with a rubber sponge.
However, environmental temperature at which the conventional polyurethane foam produced by a mechanical froth method can be used as a sealing material is from about 70 to 80° C., and compressive residual strain is large at a temperature higher than the temperature. For this reason, it was difficult to use the conventional polyurethane foam as a sealing material for a long period of time in a neighboring region of a heat source.
(1) A method of increasing intermolecular crosslinking density and (2) a method of using a compound having a functional group with high cohesive property, such as an ester group or a phenyl group, are known as a method for improving heat resistance of a polyurethane foam.
However, in the method of (1), when intermolecular crosslinking density is increased, hardness of a polyurethane foam becomes high, thereby decreasing tearing strength, and as a result, such a polyurethane foam is not suitable for use as a sealing material. On the other hand, in the method of (2), in many cases the compound having a functional group with high cohesive property, such as an ester group or a phenyl group, is solid or waxy at room temperature, and even if the compound is liquid at room temperature, many compounds are that raw materials thereof have high viscosity. For this reason, where the compound is added in a large amount to the polyurethane raw material, there is a possibility in a mechanical froth method that defects such as voids (also referred to as pinholes) are generated in the polyurethane foam. Furthermore, even in any of (1) and (2), the polyurethane foam obtained is likely to have high hardness, and as a result, such a polyurethane foam was not practical as a sealing material.